Monoclonal antibodies have become an increasingly important class of therapeutic molecules for numerous indications, including cancer, inflammatory diseases and viral infections. By specifically binding to their targets, such as cytokines in circulation or receptors on the cell surface, antibodies can either block or activate certain biochemical steps. In addition, upon binding to their targets, such as foreign organisms or cancer cells, antibodies can also recruit other effector cells from the immune system to the targets, which can lead to the destruction of the targets.
Most recombinant antibodies are used in the IgG format: a “Y”-shaped molecule with two antigen binding fragment (Fab) arms connected to the Fc fragment by a flexible hinge region. The bivalent nature of IgG offers several functional advantages over the monovalent antibody such as Fab. First, by the avidity effect, IgG, which has two antigen binding moieties, binds to its target more tightly than does a monovalent Fab molecule. This typically can be translated into a much higher activity in vivo. Secondly, compared to the Fab fragment, the IgG has a longer serum half-life in mammals, the result of having both a large molecular size, which prevents clearance in the kidneys, and the ability of its Fc region to bind to the neonatal receptor (FcRn) to avoid proteolysis in the endothelium, using a salvage pathway (Junghans, Immunol. Res. 16(1):29-57 (1997)).
In addition, IgG can also mimic the function of a biological ligand by crosslinking the receptors of the ligand on the cell surface. For example, anti-Fas antibodies, similar to Fas ligand (FasL), can activate Fas-mediated apoptosis in many cell types. Another example is anti-CD3 antibody, which is commonly used to activate T cell receptors in vivo (i.e., agonistic antibody). Currently, several agonistic antibodies to the TRAIL receptors are being developed as promising anti-cancer agents as they can induce TRAIL-mediated apoptosis. The biological functions of these agonistic antibodies depend on the bivalency of the IgG format.
The monovalent form of the same antibody, such as Fab fragment, typically fails to work as an agonistic antibody.
For certain therapeutic targets, for example, TNF receptors and the other members of the TNF receptor family, however, the activation of cell surface receptor by antibodies crosslinking is not desirable. TNF receptor family members have been shown to play important roles in many physiological and pathophysiological conditions in human, which make them attractive targets for drug intervention, in particular, by antibody-based therapeutics. However, it is difficult to develop an antibody to the members of the TNF receptor family that is purely antagonistic. The difficulty is mainly due to the potential risk of having an antibody that is both antagonistic and agonistic, in particular for a bivalent antibody such as a full IgG.
Although the monovalent forms of antibody, such as Fab, are free of agonistic activity, it is impractical to be used in vivo due to its short half-life. To overcome this problem, several strategies have been developed in the art, including fusing Fab to large molecules such as serum albumin, and by pegylation of Fab (Leong, S. R., Cytokine, 16(3):106-19 (2001). These approaches, however, are far from being optimal, due to decreased bioactivity and accumulation of PEG molecules in the kidney.
Thus there remains a considerable need for improved methods to develop an antagonistic antibody that has long half-life in vivo but is free of agonistic activity. The present invention fulfills this need by providing a heterodimeric polypeptide which functions as a monovalent antibody.